as i don't have time just now i have passed this on for someone to help you.please bear with us.

YaBB install help video1. what yabb forum are you running and the url2. describe in as much detail as you can what happens and also post screenshots if you can3. please be patient we live in different time zones and have other commitments but we will help youas bill and ted say (Be excellent to each other)

The fact that any emails get sent at all means the problem probably isn't with your YaBB mail set up. There's something being flagged and rejected by the mail server. (But you know that.)

Formmail has fixed 'from' and 'to' email addresses. YaBB's Mailer.pl has a 'fixed' from (the webmaster's email address usually) and variable 'to' which might cause emails to be rejected as spam assuming the host has some sort of email filtering activated.

Have you tried changing the 'webmaster email' in Advanced Settings -> email to the one being used in formmail?

If you only have one solution to a problem you're not trying hard enough!